1. Field of the Invention
The present invention relates to an image forming method wherein an electrostatic latent image formed by electrophotography, electrostatic recording etc. is visualized through the steps of development, transfer and fixation to give a suitable image.
2. Description of the Related Art
A method of visualizing image information via an electrostatic latent image, such as an electrophotographic method etc., is used in various fields. In the electrophotographic method, an electrostatic latent image on an electrophotographic photoreceptor (electrostatic latent image bearing body, also referred to hereinafter as “photoreceptor”) after a charging step, a light exposure step etc. is developed with a toner for electrostatic latent image development (also referred to hereinafter as “toner”) and visualized through a transfer step, a fixing step etc.
A process of producing a toner by an emulsion-polymerization aggregation process in which the shape and surface structure of the toner are intentionally regulated has been proposed as a countermeasure to the broad distribution of particle sizes, irregular shapes, and insufficient durability of performance in a process of producing a toner by a kneading-pulverizing process (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 63-282752 and 6-250439).
The emulsion-polymerization aggregation process generally means a process wherein a resin dispersion is prepared by emulsion polymerization, while a colorant dispersion having a colorant dispersed in a solvent is prepared, and the resin dispersion is mixed with the colorant dispersion, to form aggregated particles having particle sizes corresponding to the toner particle size, and then the aggregated particles are coalesced and united by heating to form a toner, and there is proposed a means of realizing more accurate regulation of a particle structure by arbitrarily regulating the internal structure of the toner from an inner layer to a surface layer (see, for example, Japanese Patent No. 3141783).
By the methods easily realizing a smaller diameter of toner and accurate regulation of a particle structure, image qualities in an electron photograph are significantly improved, and simultaneously higher reliability can also be achieved.
As digitalization and coloring advance in recent years, an image forming method by electrophotography using the toner and developing agent technology described above is now applied in some fields of printing, and coming to be practically used in the graphic arts market, for example, in on-demand-printing. The graphic arts market is defined as a market whose targets are business and divisions involved in producing printings, including tracing, copying, and reproduction, which is a business relating to production of prints employing a large-scale production system, of a small number of created printings such as wood-cut prints, of original writings and paintings.
However, when compared with the original and orthodox conventional printing, the electrophotographic methods satisfies the feature of on-demand-printing as non-planographic printing, but there remain various problems to be solved for improving the market value as producer goods of products particularly in the field of graphic arts from the viewpoint of performance in respect of color regeneration, resolution, image qualities, typically gloss, feel of the material, uniform qualities in the same image, and duration of image qualities in continuous printing for a long time.
For example, the resolution tends to be limited not only by an image processing system, a photoreceptor, and a system for light exposure, but also by the particle diameter and distribution of toners, but it is a serious technical problem to use small-diameter toners effectively and reliably in the steps of charging, development, transfer, fixation and cleaning.
To solve these problems, it is necessary to develop a carrier for uniformly charging the small-diameter toner, design of a charging blade and a charging roll, a development system for achieving higher image density without background stain, a transfer system for realizing accurate transfer at high efficiency, a fixing system for various kinds of paper compatible with the small-diameter toner, and a cleaning system for removing the small-diameter toner completely from the surface of a photoreceptor or an intermediate transfer material (transfer material) to realize stable image qualities.
For improving uniformity of a single image and for solving defects, it is important to regulate uniformity of the development performance of a developing agent in an image forming system. A highly durable developing agent with less dependence on environments such as temperature and humidity, which shows stable charging in continuous printing of several thousands copies of uniform images and keeps stable and uniform development, is necessary for meeting the demand for maintainance of image qualities in the market of printing.
As an electrophotographic photoreceptor for the development system, on the other hand, a photoreceptor using an easily producible, organic photoconductive material as a photosensitive layer is proposed and practically used. The photoconductive material is classified roughly into positive hole transporting one and charge transporting one, and as the organic photoconductive material, the positive hole transporting material can easily achieve higher performance, and thus a negatively charged laminated photoreceptor using an positive hole transporting material in a charge transporting layer laminated on the surface of a charge generating layer is widely used.
In the charge transporting layer, polycyclic aromatic compounds such as anthracene and pyrene, nitrogen-containing heterocyclic compounds such as carbazole and imidazole, hydrazone derivatives, stilbene derivatives, triphenyl amine derivatives, tetraphenyl benzidine derivatives etc. are used as the charge transporting materials, and as the materials, compounds having a large number of aliphatic double bonds in addition to aromatic rings are generally used to improve charge transportation. However, such aliphatic double bonds are easily oxidized, and in particular, the surface of the photoreceptor is very liable to oxidized with strongly oxidizing ozone, nitrogen oxides etc. upon corona discharge etc., resulting in a change in potential, sensitivity etc. to deteriorate the performance of the photoreceptor.
One the other hand, there are reported electrophotographic photoreceptors to which antioxidants, stabilizers etc., for example a compound having a triazine ring and a hindered phenol skeleton, trialkyl amine, an aromatic amine compound, an amine compound, an amide compound, and hindered amine or hindered amide compound are added to prevent oxidization and deterioration of the surfaces of the photoreceptors (see, for example, JP-A Nos. 62-105151, 63-4238, 63-216055, 3-172852 and 10-282696).
However, the toner used in visualization in the development step of visualizing the above electrostatic latent image is blended usually with external additives, for example inorganic metal oxides such as silica and titania for the purpose of improving and regulating powdery flowability, charging etc. or with similar inorganic metal oxides as internal additives for the purpose of improving fixation, or with magnetic powders such as ferrite especially in the case where the toner is a magnetic toner, so that these metal oxides collide with and adhere to the photoreceptor at the time of development thereby shaving the surface layer of the photoreceptor. Further, the toner, inorganic metal oxides etc. remaining on the surface of the photoreceptor in the cleaning step are pushed by a blush or a blade against the surface of the photoreceptor, thereby further shaving the surface layer of the photoreceptor. The surface layer of the photoreceptor undergoes gradual oxidation and abrasion repeatedly, resulting in a deterioration in the performance of the photoreceptor. This tendency is significant particularly in the filed of graphic arts where the amount of the toner used at one time is high.
Improvement in the abrasion resistance of the charge transporting layer on the surface of the photoreceptor is attempted by usually incorporating not only the charge transporting material but also a binder resin such as polycarbonate resin, polyacrylate resin and polyester resin, and by further providing the surface of the charge transporting layer with a surface protective layer of e.g. a polysiloxane compound having relatively high hardness (see, for example, JP-A Nos. 2001-154390, 2002-62777 and 2002-221886). However, inorganic metal oxides used as external additives on the toner surface are usually fine particles of 20 nm or less having higher hardness than the binder resin and the surface protective layer, so that durability can be improved to a certain degree but is not fundmentally improved at present.
Further, there is a method wherein the adhesion property of external additives to the toner is controlled to regulate the amount of the free external additives for the purpose of improving the durability of the photoreceptor, or the adhesion strength of external additives to the surface of the toner is increased to a certain degree to prevent the external additives from becoming spent (see, for example, JP-A Nos. 9-179467 and 2002-62683). However, this method leads to a reduction in the original effect of the external additives of improving the flowability of the toner, thus causing problems such as blocking of the toner in a development device.
As the transfer system, an electrostatic transfer system is generally used in electrophotography at present, but in the case of a color image using a toner image thickened by overlapping colors, optimization for precisely regulating the behavior of the toner in an electric field is necessary in the toner material and in the transfer system in order to prevent image deterioration due to the toner scattered during transfer. As the transfer system, there are a system of directly transferring a toner image on a photosensitive drum onto a recording material such as paper and a system of transferring an image via an intermediate transfer material (transfer material).
Generally, the intermediate transfer material comprises a circulating endless belt, a part of which contacts with a photosensitive drum and another part of which contacts with a transfer member etc. in an image forming device. In transfer via the intermediate transfer material, a toner image formed on the surface of the photosensitive drum is transferred at a primary transfer position onto the intermediate transfer material, and the toner image transferred onto the surface of the intermediate transfer material is then delivered by a circulating belt onto a secondary transfer position and transferred at the secondary transfer position onto a recording material such as paper.
The material of the endless belt used as the intermediate transfer material includes, for example polycarbonate resin, polyvinylidene fluoride resin, polyalkylene phthalate resin and an electroconductive material mixed with resin, and as the material excellent in mechanical characteristics, thermosetting polyimide materials are proposed (see, for example, JP-A No. 63-311263).
However, the surface of these resins is generally low in strength and liable to be abraded or scarred. In particular, fine particles of metal oxides such as silica, titania etc. are added as external additives to the surface of the toner, and by transfer of the toner from the photoreceptor at the time of primary transfer to the intermediate transfer material, the surface of the intermediate transfer material is scratched by the external additives. Even if scratches generated in each transfer may be very small, the scratches are enlarged by repeated transfer, resulting in problems such as uneven transfer, streaks and insufficient image density at the time of transfer. Since the amount of a toner consumed particularly in the field of graphic arts is large and the amount of the toner transferred in each transfer is also large, the surface of the intermediate transfer material is liable to be further abraded and scratched.
On the other hand, a toner obtained by the conventional kneading-pulverizing process wherein thermoplastic resin together with a pigment, a charging regulator, a releasing agent etc. is melted, kneaded, cooled, finely pulverized and classified, has an indefinite shape and an indefinite surface structure, and depending on the ability of the used materials to be pulverized and conditions of the pulverizing step, the shape and surface structure of the toner are slightly changed, thereby making intentional regulation of the shape and surface structure difficult. Further, the toner obtained in the kneading-pulverizing process is one having a storage of elastic modulus, which will be described below, at 160° C. (G′(160)) of 700 Pa or higher.
Accordingly, there is demand for an image forming method free of defects in images with excellent durability of a photoreceptor to solve the problems described above.